The present invention relates to a honeycomb structure for use as a catalyst carrier for purifying automobile exhaust gas. More particularly, the present invention relates to a thin-walled honeycomb structure that has thin partition walls, a light weight, a small loss of pressure, and an improved mechanical strength. All improvements result by virtue of reinforcing a circumferential portion of the honeycomb structure, thereby preventing damage to the honeycomb structure during its manufacturing or its handling. The invention relates to a method for reinforcing such a thin-walled honeycomb structure.
There has conventionally used, as a carrier for catalysts of automobile exhaust gas(hereinafter referred to as a catalyst for purifying exhaust gas), a ceramic honeycomb structure having (a) numerous cell passages defined by a plurality of partition walls and (b) a circumferential wall surrounding the cell passages(hereinafter referred to as a honeycomb structure). That is, the conventionally used exhaust gas catalysts have been produced by coating a honeycomb structure with a γ-alumina to form a layer thereof, and loading a catalytic component such as a noble metal or the like into
More concretely, a honeycomb structure has usually been produced by the steps of extrusion-molding a material, which mainly becomes cordierite when it is fired, through a nozzle having lattice-like slits so as to form integratedly a honeycomb portion and a circumferential wall, and subsequently drying and firing the thus molded article. The obtained honeycomb structure is transferred to a step of forming the catalytic layer by first coating the inner surfaces of numerous cell passages with γ-alumina so as to form a γ-alumina layer thereon, and then loading a noble metal component, as a catalytic component, such as platinum, rhodium, or palladium into inner surfaces of pores of thus formed γ-alumina layer. Then, the honeycomb structure is subjected to a baking treatment at a temperature of about 600° C. so as to back the catalytic component thereon to give a catalyst for purifying exhaust gas. The resulting catalyst for purifying exhaust gas is housed in a metallic container with the aid of a cushioning material. The metallic container, i.e., a converter is connected to an exhaust pipe by means of welding, bolting, or the like, to set up an engine for an automobile, etc.
Regulation on exhaust gases have become stricter year by year, especially in developed countries, due to environmental problems. To cope with these stricter regulations, an everlasting improvement in purification ability is required for a catalyst for purifying exhaust gas. On the other hand, a desire to lower fuel charge and increase output of power has been evident in the fields of engine development. Because of such a situation, the reduction in pressure loss during operation has been required in the case of the catalyst for purifying exhaust gas. In the case of the catalyst for purifying exhaust gas, so as to solve the problems mentioned above, there has become evident such a strong movement that the improvement in the performance of the catalyst for purifying exhaust gas at the time of warming up of the engine has been tried by increasing a passage area of the cell passages so as to reduce pressure loss. Furthermore, lightening the weight of the catalyst for purifying exhaust gas itself has been undertaken so as to reduce its heat capacity by making the partition walls thin without decreasing the number of cells as well. Hitherto, a honeycomb structure having partition walls of 0.15 mm or more tick was most popular. However, a honeycomb structure having partition walls having a thickness of 0.13 or less, particularly, 0.11 mm or less, has recently come to be popular.
However, to make partition walls of a honeycomb structure thinner causes a problem that the extremities(hereinafter sometimes referred to as a corner) of circumferential portions of the honeycomb structure are often broken during manufacturing, handling, or conveying the honeycomb structure, or housing the honeycomb structure into the container for converter so as to set it in an engine because the structural strength of the honeycomb structure is consequently decreased, particularly in the circumferential portions of the honey-comb structure. This phenomenon becomes evident when the partition walls of the honeycomb structure become thinner. This is because damage occurs in a honeycomb structure more frequently when partition walls in a honeycomb structure are thinned. Note that the honeycomb structure is liable to break when an external force such as a mechanical shock is applied thereto during transportation or the like, even in the case of the honeycomb structure having thicker partition walls. This potential breakage occurs because a ceramic material is inherently brittle. Thus, the breaking of the honeycomb structure was also occasionally reported. Because of its very low frequency of occurrence, however, it has not particularly been regarded as a problem.
Further, the frequency of the deformation in partition walls during extrusion-molding has remarkably increased as partition walls become thinner; while in the case of conventional honeycomb structures having partition walls that are 0.15 mm or more thick, such a problem is not so serious. This is because the circumferential wall has a thickness of at least 0.3 mm, and therefore, the strength in the circumferential portion can be ensured to a certain degree. The deformation in partition walls occurs mainly at the vicinity of the circumferential walls in the circumferential portions. This deformation is due to the failure to attain uniform extrusion-molding because of the unbalance in the fluidity of raw material between the honeycomb portions and the vicinity of the circumferential partition walls, when the circumferential walls are thicker than partition walls. Such thickening ensures strength in the circumferential portion.
A similar phenomenon when using cordierite to deform the partition walls is also observed when a ceramic material such as alumina, mullite, silicon nitride, silicon carbide, or zirconia is subjected to an extrusion-molding. This phenomenon occurs because, as a starting material, a material prepared by mixing and kneading the starting material with water and a binder is used as well. Since the deformation in partition walls is mainly attributed to buckling derived from compressive load, a similar problem is also observed in not only a honeycomb structure having square cells, but also in a honeycomb structure having a rectangular, triangular, or hexagonal shape.
Some proposals have been made to solve the various problems caused by making partition walls thinner in a honeycomb structure. First, it has been proposed to reduce the circumferential walls from 0.3 mm to 0.1 mm. Thus, the thickness of the circumferential walls approximates the thickness of the partition walls, thereby improving the balance in the flow amount of raw material during molding. In this case, however, the strength of the circumferential wall is not sufficient. In other words, when circumferential walls are too thin, breaking starts at the circumferential walls due to insufficient rigidity. A circumferential wall thickness of at least 0.1 mm, desirably at least 0.15 mm, is sufficient just to house the honeycomb structure in a container kept under a uniform and static external pressure. However, such a circumferential wall thickness is not sufficient to resist external pressure, such as mechanical shock during transport or the like.
On the other hand, it has been proposed that the strength against mounting pressure of the circumferential walls would increase if the circumferential walls are thickened. Thus, a cordierite honeycomb structure having square cells, a partition wall thickness of 0.11 mm, and a circumferential wall thickness of at least 0.4 mm was prepared so as to increase strength. Contrary to expectation, however, it was found, as a result of an isostatic strength test, that the honeycomb structure was not improved in strength and had a tendency of deterioration in strength. An investigation was made to clarify why the isostatic strength could not improve when only the thickness of the circumferential walls was made thicker. As a result, it has been found that partition walls (ribs) around cells in the circumference in a molded article are deformed to a great extent just after extrusion-molding, and that the number of deformed partition walls increases as the circumferential wall is made thicker.
If the circumferential wall is thickened, the amount of a raw material passing through slits for forming the circumferential walls increases when the raw material passes through slits of the nozzle upon extrusion-molding. As a consequence, partition walls around circumferential cells are pulled toward the circumferential walls, or the circumferential walls press the internal partition walls of the honeycomb structure. Thus, it has been evident that the remarkable gap in the unbalance between a flow of the raw material for the circumference wall and a flow of the raw material for the partition walls is considered to be a major cause. Further, the thinning of the partition walls brings buckling deformation more easily. In addition, the circumferential wall and partition walls in the circumferential portion are deformed by the weight of the honeycomb structure itself at the time when a honeycomb structure is supported by a jig on the surface of the circumferential walls right after extrusion-molding. These are also considered to be the main causes.
If the internal partition walls of the honeycomb structure are molded straight, the breakage of the honeycomb structure starts owing to the compression of the partition walls when pressure is applied to the honeycomb structure from the surface side of the circumferential wall. This is because the internal portion of the honeycomb structure is theoretically the center of compressive stress. However, in the case where partition walls at the vicinity of the circumferential portion are deformed, or when the circumferential wall is extremely thin, bending stress, i.e., a tensile stress, is generated at the position of partition walls of the honeycomb structure. Since the ratio of tensile strength to compressive strength is generally as low as about 1:10, the honeycomb structure starts to break if it has deformed partition walls when even only very low strength is given thereto.
On the other hand, even if the circumferential wall can be considerably thickened at the time of molding, a great difference in heat capacity exists between the honeycomb portion having thin partition walls and the thick circumferential wall, thereby lowering thermal shock resistance of the honeycomb heater.
In order to solve the problems derived from an extreme difference in thickness between the honeycomb portion and the circumferential portion, the following proposal has been made; a molding is carried out, with adjustment of a raw material flow at the time of extrusion, by making partition walls in the circumferential portion and the circumferential wall thicker to enhance pressure resistance in an axial direction of the honeycomb structure and molding is carried out by adjusting. However, since the adjustment of balance is very subtle when this means is used, it becomes more difficult to suppress deformation in the partition walls as the circumferential wall becomes thicker. Furthermore, the thicker circumferential portion has a greater influence on its own heat capacity. In this case, the temperature difference between the inside and the outside of the circumferential wall increases; thereby thermal shock resistance of the honeycomb structure inevitably decreases. Furthermore, since such means brings about an increase in weight of the honeycomb structure, the performance of the catalyst after an engine is warmed up is lowered. Furthermore, it is not so preferable due to the pressure loss problem.
Thus, numerous studies have conventionally been made so as to solve the various problems caused in accordince wtih thinning of partition walls in a honeycomb structure. However, the problems have not been solved yet. Under such conditions, the present inventor paid attention to reinforcement of the circumferential portion, particularly its edge portion, in the honeycomb structure.
There has been known a method of reinforcing a honeycomb structure. In the method, a coat layer is formed by applying a ceramic material as a reinforcing material for reinforcement of the circumferential portion on the circumferential wall without unitarily forming the thick circumferential wall by extrusion molding, or filling the reinforcing material into cells in the circumferential portion. However, this method has drawbacks such as, for example, the reduction in the thermal shock resistance of the honeycomb structure during the practical use, the occurrence of the detachment in the coat layer due to cracks generated in the coat layer, the formation of the cracks that sometimes reaches to the honeycomb structure due to the shrinkage of the coat layer itself caused by a high temperature generated during engine operation, and the like.
There has been known another method, in which a resin is coated on the surface of the circumferential portion. The aim of this method is to prevent loading of catalyst on the surface of the circumferential wall, and therefore, a water-repellent resin film is formed on the surface of the circumferential wall by using a resin material having low strength such as vinyl acetate, fluororesins, or silicone resins. Thus, in the method it is not intended to reinforce the circumferential portion of the honeycomb structure. Therefore, the film formed is thin since this film is formed not so as to reinforce the circumferential portion. In fact, the thin film does play any active role in the reinforcement of the circumferential portion.